In the many chemical industries, maintenance and capital improvements occur on a scheduled basis. Operating units including various types of vessels from tanks to towers to heat exchangers to distillation columns and other types of equipment are maintained as part of an effort called a turn-around.
During a turn-around, an operating process vessel cannot be merely shutdown and drained before personnel entry for maintenance. The vessel must first be decontaminated of any residual materials in it, which may render the vessel harmful for workers to enter.
During a turnaround, and before internal mechanical maintenance is performed of any kind, a cleaning must take place which frees all the internal surfaces of the process equipment from contaminants. These internal surfaces may include the walls of the vessel cylinder, the tops and bottoms of trays, packing sections (loose or fixed), spargers, pump-around piping, and especially the bottom third of the vessel. The bottom section is typically very difficult to dean since heavier chemicals tend to settle and accumulate in this area.
The contaminants removed are often hydrocarbons. These hydrocarbons will vary in size, length, molecular weight and structure. The industry commonly categorizes hydrocarbons into Light ends, Medium cuts and Heavy cuts. Light ends would be cuts like methane, propane, ethane, and the like. Medium cuts would include kerosene, gasoline, and diesel, among others. Heavy cuts would include lubricants, waxes and asphalt.
There are several methods for cleaning pressurized vessels known in the prior art including liquid circulation of aqueous solutions of cleaning chemicals and the injection of cleaning chemicals into steam sparged through the equipment to effect decontamination. The prevalent process used for decontamination is through steam injection, also called vapor-phase cleaning.
In vapor-phase cleaning, a chemical is injected into a steam stream that is sparged through the equipment over a period of time. The chemicals used for this purpose are often termed as degassing products intended for use in the removal of the hazardous gases, such as hydrogen sulfide, ammonia and light hydrocarbons such as benzene. While deemed degassing chemicals, the process is primarily intended to remove residual liquid hydrocarbon oils from the equipment. In doing so, the residual oils that are the source of gas generation within equipment, are removed thereby allowing for more rapid gas elimination via steam pressure.
Rapid removal of the residual hydrocarbon liquids is an important component in the time efficient application of the degassing process. Otherwise, residual liquid hydrocarbons volitalize and become the source of continued evolution of hazardous vapors. Without the removal of the residual hydrocarbon liquids, otherwise known as degreasing, the decontamination process often would require long periods of time, days even, before the equipment could be authorized for safe entry.
While simple and relatively inexpensive, the cleaning performance of high-pressure steam is very poor. By itself, the steam process will not reduce the hydrocarbon gas. Therefore, the vessel is usually opened to the atmosphere until the hydrocarbon gas has volatilized and moved out of the vessel. This airing out process may take as long as two days.
Another method incorporates liquid cleaning with a caustic solution. Caustic solution cleaning begins like the basic steam cleaning method—with a reduced crude wash. After the reduced crude wash, caustic or high-pH chemicals are circulated through the vessel. Due to the high pH of the caustic chemical, effluent generated during the caustic wash must be neutralized with an add to neutralize the pH before the significant quantities of effluent are sent to a wastewater-treatment plant for processing. Additional processing may be required if the caustic chemicals contain phosphates, silicates, or other chelating agents that can interfere with the waste-treatment process.
Yet another method involves an organic solvent wash. This method, like the first two, begins with a reduced crude wash. Next, organic solvents are circulated through the vessel from top to bottom. Although these organic solvents may satisfactorily remove oils, they do not have the solvency strength necessary to thoroughly clean the vessels while in a liquid phase. Solvent circulation can last as long as 24-48 hours. After the liquid phase cleaning, a water rinse is used to remove organic contamination from the vessel. Since organics by nature are not water soluble, rinsing with water is time-consuming, inefficient, and very difficult. Additionally, it is extremely difficult to determine whether these potentially harmful organics have been completely removed by the rinse process.
In summary, each of these prior art methods require that the vessel be exposed to the atmosphere to remove harmful gas and then manually cleaned to remove contamination.
One alternative procedure envisages inertisation of the atmosphere inside the tank before degassing, for example by introducing nitrogen or argon into the tank through tank openings.
As well as being expensive this operation may considerably lengthen degassing times. In fact, inertisation requires the prior sealing of all the gaskets of the tank, including the legs supporting the mobile roof. The vaporisation of the nitrogen, the dispersion of the nitrogen inside of the tank and periodic tests of control to check the level of inertisation achieved, all add to the length of the degassing procedure.
U.S. Pat. No. 6,893,509 B2 (Sears et al., 2005) teaches a process of injecting a terpene and a surfactant package into high-pressure steam, and introducing the steam and chemistry mixture into the equipment to clean its inside surfaces.
U.S. Pat. No. 8,480,812 (Nath et al., 2013) teaches a cleaning agent containing one or more solvents, such as terpenes or other organic solvents. The cleaning agent is injected into contaminated equipment, along with a carrier gas, in the form of a cleaning vapor. The carrier gas may be nitrogen or other inert gases.
WO 2011145122 A1 (2011, Rancich) teaches a method of introducing an atornised aqueous mixture which interacts chemically and/or physically with VOC (volatile organic compounds) in the gaseous state inside a tank, so as to encourage them to pass from the gaseous phase to the liquid phase, and to remain in the liquid phase. Aqueous mixture comprises one or rn surfactant substances and chelating substances that aid in VOC absorption.
U.S. Pat. No. 8,491,721 (Ortega et al., 2013) provides a method for cleaning storage tank by mixing a high-powered emulsifying agent with the residual of water, solids, and trace amounts of hydrocarbons to substantially emulsify the hydrocarbons.
U.S. Pat. No. 6,905,577 (Salama, 2005) teaches the use of an electrical corona discharge reactor capable of producing ozone for the oxidation of VOC's, as well as other undesirable compounds such as H2 5, NH4, mercaptans, and chlorinated solvents which can be present in gaseous effluents.
U.S. Pat. No. 5,776,257 (Arnold et al., 1998) Teaches a method of removing VOC's from a tank by repetitive spraying of diesel mist which is allowed to settle. This permits the diesel spray to act as a sponge and remove gaseous airborne VOC's in the tank.
U.S. Pat. No. 20130087511 A1 (Ledebuhr et al., 2013) teaches VOC removal from floo,vback water stored in holding tanks. A portion of the contained water is routed and pumped to an atomizer at the top of the tank in the space above the contained water thus creating a mist of very fine droplets that capture VOC vapors.
U.S. Pat. No. 5,356,482 (Mehta et al., 1994) teaches a liquid-steam method using terpenes to detoxify the insides of a vessel to remove dangerous and explosive gases. Specifically, it involves use of steam acids alkalies or wetting agents to decontaminate the vessels.
U.S. Pat. No. 5,425,814 (Krajicek et al., 1995) teaches using an aqueous solution at an elevated temperature, of an extractant, such as a terpene, and a surfactant mixture which extracts and traps contaminants.
U.S. Pat. No. 6,872,263 (Jansen et al., 2005): the present invention employs a naturally occurring organic solvent as a cleaning agent injected directly into one or more high-pressure steam lines already present in the refinery's system. The cleaning agent and stream mix as vaporized and allowed to enter into the equipment where upon it cleans all surfaces inside the equipment.